Graphene is a monolayer of carbon atoms densely packed in a benzene ring structure and forms blocks of various structures, such as carbon black, carbon fiber, carbon nanotubes and fullerene.
Such graphene is an intriguing 2D flat material consisting of monolayer carbon atoms, and the free and fixed individual graphene sheets possess distinct properties and have promising applications in nanoscale engineering and the fabrication of nanoscale systems.
Namely, the distinct properties of graphene make it very promising in various applications such as field-effect transistors, lithium-ion batteries, hydrogen storage, molecular sensors, actuators, and reinforcing fillers in high-performance polymer nanocomposites.
Since producing graphene by mechanical exfoliation (peeling off method) was first reported, a wide range of techniques for synthesizing graphene, which can be divided into top-down and bottom-up processes, have been developed.
In bottom-up methods such as thermal chemical vapor deposition (CVD), graphene is synthesized by allowing graphene precursor particles to gradually grow in size, that is, graphene grows because of epitaxial growth on substrates or graphene platelets are grown by CVD. Theoretically, it is possible to control the size, shape, size distribution and agglomeration using the bottom-up processes, but it is actually very difficult to realize this control. Although CVD allows high-quality graphenes to be produced, it causes environmental concerns about the emission of PAH (polyaromatic hydrocarbons) (see the instructions of the California Environmental Protection Agency (EPA)).
The more common top-down process includes stripping individual graphene sheets off of graphitic structures such as expanded graphite and carbon nanotubes. Many techniques for obtaining monolayers have been developed, including physical methods of hand-stripping individual graphene layers using Scotch tape, electrochemical methods using an ionic liquid as a solvent, and the thermal and chemical exfoliation of expanded graphite oxide.
In such top-down processes, graphitic microstructures such as graphite, carbon fibers and carbon nanotubes are used as starting materials, individual graphene layers are extracted or peeled either by physical, electrochemical or chemical methods.
Particularly, exfoliation of individual graphene sheets from graphene oxide is the most common method, and many techniques capable of successfully obtaining monolayered graphenes have been reported, but such methods for producing graphene platelets from graphite are carried out using oxidizing agents such as sulfuric acid/potassium permanganate (KMnO4) and are followed by a reduction reaction with any one of hydrazine and alkali or solvent etching.
Very excellent methods for the production of graphene that have most recently become known are based on the longitudinal unzipping of multiwalled carbon nanotubes (MWCNTs) by physical or chemical methods. These novel techniques are attractive because of their convenience and can provide graphene sheets having a previously defined shape, but have a problem in that a number of nanoholes can occur in graphene layers due to excessive, localized oxidation.
Most chemical methods known to date extensively rely on the usage of strong oxidizing agents such as sulfuric acid/potassium permanganate (KMnO4), carboxylic acid or formic acid and on the excessive use of environmentally harmful organic solvents to create additional exfoliation.
In addition, the above-described techniques based on longitudinal unzipping are based on the extraction of graphene from carbon structures, in which the graphene layers are arranged in a perfect AB stacking structure. However, there are many carbon structures in which the packing of graphene layers is not perfect AB stacking. Rather, the structures are fundamentally composed of graphene crystallites which are slightly randomly arranged with respect to their common vertical axis without an almost perfect orientation. Biscoe and Warren proposed the term “turbostratic structure”. Carbon black, carbon fibers and vapor growth carbon nanofibers (VGCNFs) all have the turbostratic structure. In graphitic structures having this turbostratic structure, the graphite crystallite layers are randomly rotated with respect to each other, even though they still have c-direction orientation.
Accordingly, there has also been a need for a method of preparing graphene sheets from graphene layers constituting a turbostratic graphitic structure.